Diana was a flawed person. Charles was and IS a flawed person. Both caught up in a bad situation. Charles is hemmed around by "duty." He's been taught his whole life to do his "duty." So when he is told by his father to marry that girl or let her go, he does the "honorable" thing and marries her. Was he also incredibly selfish that he wasn't going to let his mistress go either? yes. Diana wanted the fairytale princess role to make her whole life perfect because her parent's nasty divorce had scarred her horribly. So she said yes when Charles proposed even though it was clear they had nothing solid to build a marriage on. Then she was just thrown in the deep end and expected to be the "perfect" Princess right from the start. Did she manipulate that image to her own ends? Oh hell yes.
William and Kate get a lot of grief for not being full time working Royals and doing so few engagements when the Queen is so old and Charles is not exactly a spring chicken. But the Royal Family learned at least ONE thing from the experience with Diana, and to a lesser extent Sarah Ferguson. Don't expect royal wives to just jump in with no support. Let them be a family for awhile. Let them get a solid base as a family first before they have to be "on show" all the time. It will help in the long run.
The Royal life is not all the lawn is mowed and your travel is planned for you. There is a LOT of work that goes into it. Those Christmas decoratioins that go up each year right on time? There's a theme each year and the Queen has to approve it. The Queen reads her red box every day -- even on "vacation." Those trips --most of them are business trips with the schedule packed with meeting people and going places chosen by others. It's not "Hey I'd like to go to Bora Bora for 2 weeks and go snorkeling." Yes they get more vacations than the rest of us do, but they work a lot more nights and weekends than the rest of us do too. Those fabulous clothes? How many times have we on here criticized something that one of the Duchesses wore to an official engagement? At least when you or I go out, even some place fancy, no one picks apart our clothes, our shoes, our make up, our jewelry. No one is watching our every move and listening to our every word to criticize. Those servants that are around to get the cufflinks, put out the clothes, etc? Means there is NO privacy. They knew Diana was having an affair before Charles did. They knew Charles was still sleeping with Camilla before Diana did. Because again, no privacy.
To sum up -- Charles and Diana - human with their good and bad points. Being royal - pros and cons. Like more people and life in general.